1. Field of the Invention
The present invention relates to power operated doors and more particularly to logic circuitry for controlling the opening and closing thereof.
2. Description of the Prior Art
Power operated doors are conventionally equipped with entry and exit mats which respectively close switches when either or both of the mats is stepped on. It is conventional to use direct current voltage applied to the mat switches which eventually causes electrolysis of the contacts, an increase in contact resistance resulting. Also, it is conventional to use high current AC which causes contact sparking. Contact resistance is also affected by reason of contamination due to corrosive atmosphere, water and other residue. When the contact resistance becomes too high, the mat switches become ineffective in controlling the door-operating mechanism.
It is customary to consider the contact resistance in a mat switch as being operable if it is less than 300 ohms. However, the sensitivity of the door-opening mechanism has not been sufficiently reliable to assure consistent door operation should the contact resistance be above, for example, 100 ohms. In successive operations, the door-opening mechanism will activate, in others it will not. Prior art systems have fixed time delay of approximately 11/2 second.
Also, there are "solid state" prior art devices that do not have a sharp resolution feature; therefore, they have uncertain time delay periods and annoying propagation delay upon initial activation.
Prior art systems customarily utilize relay type switches in the mechanism which are not only large and expensive but require relatively large amounts of power to operate. Further, they fail to inhibit and hold due to contacts not making good contact.